1. Technical Field
The present invention relates to printing systems. In particular, it relates to improved flexographic printing plates which improve print quality by minimizing dot gain in flexographic printing systems that is caused by the compression of the flexible dots used to form the images on the flexographic plates.
2. Background Art
The commercial printing industry uses several technologies. The first technology is offset lithography, commonly referred to as offset printing. The offset printing process begins with the authors who create the written material, either actual or graphic, which is intended to be placed in the ultimate printed material. Once these materials are created, they are typically compiled into an electronic file. Once the electronic file is created, it is used to create a plate which holds an image of the data. Offset printing uses metal plates to create images. (NOTE: Older offset technology used a film negative as an intermediate step between the computer and generating a plate.)
So far, the plates have been referred to as xe2x80x9cmetalxe2x80x9d plates for ease of discussion. However, those skilled in the art will recognize that the rigid plates used in this type of lithography can be manufactured from a variety of materials ranging from high-quality aluminum at one end of the spectrum to a low quality material, such as paper, at the other end of the spectrum. The plate material selected will depend on the nature of the ultimate printed material. For example, high-quality printing, such as that used in books containing art reproductions, will require that a high-quality plate material be used. For the purpose of this disclosure, the term metal plates will be used to describe any rigid plate material.
Once the image is transferred to the metal plate, the metal plate is pressed against a xe2x80x9cblanketxe2x80x9d plate, which is typically made from a rubber or rubber-like material, and then transferred from the blanket plate to the paper. The paper is then dried in an oven and then chilled to set the ink so that it will not smudge.
In four color process printing, typically four plates are used, each plate using either red, blue, yellow or black. The use of multiple plates to create a single image requires substantial care to be made in registering and aligning the plates to ensure that the final printed image is crisp and clear. This is typically accomplished through the use of skilled press personnel.
Metal plates provide several advantages to a printer. The most important of these advantages is the high print quality which metal plates provide. The print quality provided by metal plates is very good because of the rigid nature of the metal plates. When metal plates are pressed against a print surface, the metal plates have sufficient strength and rigidity to prevent them from deforming. In turn, the resistance to deformity provides superior print quality.
A second technology used for many print applications is known as flexographic printing. Flexographic printing is a direct printing method rather than an offset printing method. Flexographic plates are typically engraved plates containing relief images which are cut into flexible plates made from rubber, photopolymer, or any other suitable flexible material. Flexography has an advantage over offset lithography in that it can print on practically any surface material. It can use fast drying inks which allow it to be used to print on what would otherwise be a difficult surface for a traditional print method such as offset lithography.
While flexographic printing provides several advantages over offset lithography, such as the ability to print on surfaces which may not be suitable for offset lithography, it also has a disadvantage in that it does not have the print quality of offset lithography in certain situations. In particular, the flexible material, which forms the flexographic plate, experiences significant compression as the dot screen percentage drops below ten percent. This is particularly apparent when the low percentage dots are adjacent to areas containing no dots. As the dot is deformed, it flattens and forms an expanded print area. This condition is commonly referred to as dot gain. The expanded print area caused by the deformed dot results in an excessive amount of ink being applied to the print surface, which in turn results in defects in the print such as darker areas or defects in coloration. It would be desirable to have a method of eliminating or reducing defects in the printed material due to dot deformation such that flexographic printing could produce print quality similar to offset lithography.
While addressing the desirability of using flexographic printing, the prior art has failed to provide a method of manufacturing flexographic plates, and a method of using flexographic plates which would minimize quality defects created by dot gain that is produced by dot deformation in the flexographic plates.
The present invention solves the foregoing problems by providing a very low percentage dot screen on a flexographic plate that prevents the plate from having the severe compression normally found adjacent to zero percent areas of the flexographic plate. The low percentage dot screen provides multiple dot screen points in the recessed areas of the flexographic plate and reduces dot gain by distributing the compressive force of the flexographic printing plate on the printing surface. The total area of the dot screen is a very low percentage of the total surface area being printed, and will not alter perceived color. The dot screen does not have a noticeable effect on color, and substantially reduces the dot gain inherent in flexographic plates.